I have worked for UPS during peak season.
It is all about time windows. There is 2 small windows of chaos at any UPS Center. Outbound and Inbound
. The rest is Delivery.
Zones = Days of ground transit.
Only certain sorting locations (hubs) get to send packages to non-sorting locations.
Those big Gray trailers (not your local, brown package car) move packages between hubs and all but the remotest "centers" once a day or to another hub.
If you are foolish enough to send a "Next Day Air" to a neighbor, the package will probably go all the way to Atlanta and back.
The USPS does similar, you just don't get tracking detailed like FedEx and UPS do.